2. INSTRUMENTAL CONSIDERATIONS

Progress in understanding the far-ultraviolet radiation from galaxies
has been more circumscribed by instrumental limitations than was the
case, for instance, in extragalactic X-ray astronomy. Fewer
long-lived ultraviolet facilities have been available, and most of
these have not been well suited for the study of galaxies. The
problems are both intrinsic and technical. Intrinsically, galaxies
are faint, extended sources. For typical elliptical galaxies, incident
far-UV photon rates per unit solid angle per unit wavelength
are typically over 50 times smaller than in the V-band. The centers
of nearby bright ellipticals produce only a few × 10-15
erg s-1 cm-2 Å-1
arcsec-2 at 1500 Å averaged over a 10"
radius
(Burstein et al
1988,
Maoz et al 1996,
Ohl et al 1998).
The paucity of high contrast spectral
features in UV hot star spectra at the spectral resolution and S/N
possible for E galaxies has also hampered interpretation.

There has never been a large area UV sky survey sensitive enough to detect
galaxies. The only all-sky survey yet made in the UV was by TD-1 in 1973
(Boksenberg et al
1973).
This has a limit of about 9th magnitude and did not include a single
galaxy or QSO. The GALEX mission
(Martin et al
1997),
now under development, will remedy this situation and produce a
survey up to 10 magnitudes fainter. For now, however, the fact remains
that the deepest survey of the UV sky is comparable
to the Henry Draper catalog of stars, made around 1900. So UV
astronomy, at least in this sense, is still 100 years behind optical
astronomy.

The technical development of UV instrumentation has been reviewed
by Boggess & Wilson
(1987,
spectroscopy), O'Connell
(1991,
imaging), Joseph
(1995,
detectors), and Brosch
(1998, surveys). UV
telescopes have been small, mostly less than 40 cm diameter. Other than the
2.4-m Hubble Space Telescope (HST), the largest UV instrument
available has been the 1-m diameter Astro Hopkins
Ultraviolet Telescope
(HUT), which as a Shuttle-attached payload had an equivalent dedicated
observing lifetime in 2 missions of only about 6 days
(Kruk et al 1995).
Observations of galaxies are difficult with the
small entrance apertures available on most UV spectrometers, for example
the International Ultraviolet Explorer (IUE)
(10" × 20") or the HST/Faint Object Spectrograph
( 1"), which were
designed for point sources. With IUE, long
exposures of typically 48 hours were needed to register far-UV
spectra of galaxies. Newer instruments are better matched to
requirements for galaxy work. HUT was the first UV spectrometer designed
specifically for galaxies, with apertures as large as 19" ×
197" (providing, however, only one spatial
resolution element). The Astro Ultraviolet Imaging
Telescope (UIT) experiment, designed for filter imaging in the
12303200 Å region, had a field of view
(40') and spatial resolution
(3") well matched to ground-based studies of nearby galaxies.
The new Space Telescope Imaging Spectrograph (STIS) offers UV
apertures up to 2" × 52", encompassing many spatial
resolution elements, and can image 25" × 25" fields with
UV photon-counting detectors and 0.05", resolution. The HST
Advanced Camera for Surveys, scheduled for installation in 2000, has
high throughput UV cameras with fields up to 30" × 30".

The quantum efficiencies of UV detectors such as cesium iodide and
cesium telluride photocathodes are only modest (1030%), and net
throughputs are further compromised by the lower reflectivities and
transmissions of UV optical components. The most widely used mirror
coating, magnesium fluoride, has a short-wavelength cutoff near 1150
Å. To obtain response to the Lyman discontinuity at 912 Å
special coatings such as silicon carbide are now available (e.g.
Kruk et al 1995),
though these do not achieve reflectances typical
of standard coatings at longer wavelengths.

Two special requirements for far-UV observations have serious
practical consequences. First is the necessity to suppress the
effects of the strong geocoronal
Ly- emission line at
1216 Å.
This is usually straightforward in spectrographs, but in photometers
or imagers the only remedy is to use blocking filters that permit response
only for 1250
Å. Second is the necessity to
suppress residual filter and detector response to long-wave
( > 3000 Å)
photons. Even though this may be only a tiny fraction of peak UV
response, it covers a wide wavelength range. Because cool
sources, such as stars with Te < 7000 K, can have
optical
f
thousands of times higher than their UV
f,
there can be serious "red leak" contamination of UV
observations. Despite considerable effort (e.g. on Wood's filters), it
has not been possible to develop fully satisfactory long-wave
blocking devices with good peak UV response. Therefore, red leak
suppression depends on the use of "solar-blind" detectors with
large photoelectron work functions, such as cesium iodide, which has very
small response for
> 1800 Å. Such detectors have been
used in most UV spectrometers but were not available in the HST Wide
Field Camera (WFPC2) or HST Faint Object Camera (FOC), both of which
consequently required careful red leak calibrations for use shortward
of 2500 Å. The effects of red leaks on
HST photometry of stars and galaxies can be dramatic and have been
discussed by
Yi et al (1995),
Chiosi et al
(1997).
The requirements for simultaneous
Ly- and red leak
suppression imply smaller bandwidths and
lower throughputs for far-UV imaging or photometry than is typical at
longer wavelengths.

Because of these technical constraints, the working "far-ultraviolet"
(FUV) band covers ~ 1250-2000 Å for imaging or
photometry, extended to about 1150 Å for spectroscopy. The
"mid-ultraviolet" (MUV) band covers ~ 2000-3200 Å (3200 Å
being both the useful sensitivity limit of cesium telluride
photocathodes and the short-wavelength cutoff of the Earth's
atmosphere). We will call the 3200-4000 Å region accessible from
the Earth's surface the "near-ultraviolet" (NUV). The 912-1150 Å
region in galaxies has been explored to date only by HUT, though FUSE
(launched in 1999) will also cover this range in brighter objects.

Unless noted, magnitudes quoted in this paper will be on the monochromatic
system, where
m =
-2.5 log F
- 21.1 and
F is the
mean incident flux in the relevant band in units of erg s-1
cm-2 Å-1; the zero point is such that
m(5500
Å) = V. Notation for colors will be, for instance, 1500-V
m
(1500 Å) - V.